1. Field of the Invention
The present invention relates to transparent laminates made from polymers having a high content of hydroxyl groups and special polyurethane-polyureas as adhesive layers (coupling agents), which laminates may furthermore bear different materials as outer layers.
Polymer films made from polyvinyl alcohol (PVAL) and from the copolymers with olefins are particularly outstanding in two valuable properties: they have a good barrier effect against oxygen and in the stretched state an outstanding orientation behaviour with incorporated foreign substances, such as iodine or dyestuffs. Due to the barrier effect, corresponding films are used as an integral component of packaging films, particularly for the preservation of foodstuffs. The orientation behaviour with incorporated foreign substances is used industrially for example in the production of polarizing films, where homopolymeric PVAL is superior to any other material.
On the other hand, a serious disadvantage of films of this type is their high sensitivity to moisture. Therefore, as a rule, industrial packaging and polarizing films with an active layer of PVAL have a multilayer construction; accordingly, depending on the intended application, PVAL films are laminated on one or both sides with protective films.
2. Description of the Related Art
In order to produce composite films of this type, it is necessary to bond the outer layer to the active layer (core layer). In the case of PVAL copolymers with a high polyolefin content (saponified ethylene-vinyl acetate (EVA) types) this bonding is achieved without any special intermediate layer, as described for example in JP 52/115,881 (1977), JP 62/46,643 (1987) and JP 62/46,644 (1987).
The lamination of saponified EVA types for producing packaging materials using special bonding layers, such as thermoplastic copolyester mixtures (EP 118,226) and modified polyolefins (JP 58/24,451 (1983}; EP 186,894), is furthermore known.
A lamination by plasma treatment of the layers which are to be bonded is also possible with saponified EVA types (JP 61/280,930 (1986)).
The production of laminates with a pure (homopolymeric) polyvinyl alcohol active layer, as is customary for polarizing films is significantly more difficult. With polarizing films of this type the lamination serves chiefly to protect the active layer, which has been doped with iodine or dyestuff, against the effect of moisture; the laminated films are additionally bonded in use, for example in displays. An old method of achieving improved moisture characteristics in PVAL films. exploits the crosslinking reaction of the PVAL matrix with boric acid. This produces only unsatisfactory results however. Better results are obtained by coating the film with adhesive and crosslinkable organic systems. Examples of systems of this type are mixtures of epoxy resins with polyamides (JP 54/133,356 (1979) and JP 54/163,051 (1979)) and branched polyurethanepolyacrylates having unsaturated groups, which are crosslinked by UV radiation (JP 56/80,001 (1981) JP 55/36,862 (1980) and JP 55/12,930 (1980)).
Normally however, PVAL polarizing films are produced in the form of permanent laminates Epoxy resins curing with amines, acid anhydrides or polyamides are described as suitable for bonding the active layer (core layer) to the outer layer (DE-OS (German Published Specification) 1,594,044; U.S. Pat. No. 4,416,946). Acrylate systems too have also been used, monomeric or oligomeric systems with vinyl groups being used, which systems can be thermally, free-radically or photochemically cured after application of the outer layer (JP 56/33,610 (1981), JP 59/58,410 (1984), JP 59/48,714 (1984), JP 58/171,008 (1983)). JP 55/155,307 (1980) moreover describes, as bonding layer, mixtures of polyacrylates with phenolic resins, which are applied in solution. Finally, isocyanate adhesives (JP 53/47,696 (1978)) and polyurethanes (JP 60/159,704 (1985)) have also occasionally been used as the intermediate layer for the lamination.
The lamination methods of the prior art have various disadvantages. The adhesive layers used for packaging films are only suitable for bonding saponified EVA types and corresponding copolymers to other olefins, so that the very good barrier effect of pure (homopolymeric) PVAL cannot be fully exploited. Other disadvantages are present with polarizing films. The films which have been provided only with a moistureexcluding outer layer must be freshly bonded by means of an additional layer during incorporation, for example in displays, since the outer layer no longer has an adhesive effect. This disadvantage likewise exists with permanent laminates having a bonding (adhesive) layer and an outer layer. The bonding (adhesive) layers (epoxy and acrylic systems) used for laminating are moreover only suitable for immediate bonding with the outer layers, since monomers and oligomers are used which only produce a stable laminate after curing. Additionally, the chemical composition of the adhesive layers must in each case be optimized in relation to the outer layer, in order to produce a composite (laminate) of high quality.
Thus, until now, no coating has existed for PVAL, which protects the material against the effect of moisture, is tack-free under normal conditions, and so allows trouble-free transport of the film bearing an adhesive layer, and nevertheless allows bonding with conventionally used outer layers at any desired time without the additional application of further adhesive layers.